Wolf-Rayet Stars

Abstract

Wolf-Rayet stars represent an evolutionary phase in the lives of massive stars during
which they undergo heavy mass loss. They are characterised by an extraordinary spectrum
which is dominated by emission lines of highly ionised elements.

Keywords: Wolf-Rayet, W-R, WR stars

Introduction

Wolf-Rayet stars are evolved, massive, extremely hot (up to ~50,000 K) and very
luminous (105 to 106 L¤).
They are extremely rare, reflecting their short lifespan.

Their surface composition is extremely exotic, being dominated by helium rather than
hydrogen, and typically showing broad wind emission lines of elements like carbon (WC
type), nitrogen (WN type), or oxygen: the products of core nucleosynthesis. The presence
or absence of hydrogen, respectively, is used to distinguish the so-called
‘late’ type WN stars (WNL) from the ‘early’ (WNE) types.

Intense stellar winds drive mass loss rates of several 10-5 up to 10-4
M¤ per year; the latter are at least three or
four times that expected for other hot, O-type or B-type stars.

"The dense stellar winds completely veil the underlying atmosphere so that an
atmospheric analysis can only be done with dynamical, spherically extended model
atmospheres, such as those developed by Hillier (1991), Hamann (1994), and Schmutz (1991).
Significant progress has been achieved in that respect so that W-R stars can be placed on
the HRD with some confidence (see e.g., Hamann, Koesterke, & Wessolowski
1993). Almost all of the Galactic WNL stars have an observable amount of hydrogen at their
surface; some have more than 50% hydrogen (Hamann et al. 1991; Crowther et al.
1995). This property of WNL stars is opposed to other W-R subtypes. The ionising spectrum
of WNL stars is very similar to what is observed in luminous O stars (Esteban et al.
1993). Despite their similar luminosities and effective temperatures, W-R and O stars
differ drastically in their masses. The most luminous O stars have current masses around
80 M¤ (see, e.g., Kudritzki et al.
1991), whereas WNL stars in binary systems have an average mass of 20 M¤ (Cherepashchuk 1991). Standard evolutionary models assume
that heavy mass loss reduces the mass of O stars, so that W-R stars are the low mass
descendants of previously massive O stars (Maeder 1990). The close relationship between
WNL and extremely luminous O supergiants is also suggested by the similar spectral
morphology of the least extreme WNL stars and the most extreme O stars (Walborn 1974;
Walborn et al. 1985, 1992)" (Nota et al. 1996, p. 384).

Characteristics

Spectra

The first spectral studies of Wolf-Rayet objects
were undertaken by the French astronomers, C. Wolf and G. Rayet, after whom the class is
named. Instead of dark absorption lines, the stellar winds give rise to strong emission
bands of highly ionised elements (Conti & Massey 1989). Lines are broadened in the
hottest subtypes, which is believed to be correlated with v¥.

The velocity of a stellar wind, initially denoted n0
where it leaves the ‘surface’ of the star, increases with distance. At some
point, far from the star, it reaches a maximum known as the terminal velocity and denoted
v¥. For hot winds (Abbot 1982) it is thought to be
about three times the escape velocity. (Also see Stellar Winds.)

Their surface composition is highly anomalous, being dominated
by helium rather than hydrogen. The latter is apparently absent in many cases. Three
sequences and a transitional phase are recognised by Langer 1990:

The spectral lines show high levels of ionisation including the presence of ionised
helium, He+, but also C+3 or N+3, which suggests temperatures equal
to or exceeding those of O-type stars.

Radii are very difficult to determine at the best of times, and
especially so where strong mass loss makes the concept of a stellar ‘surface’
problematic. However, a few estimates have been made from eclipsing binaries, such as 11 R¤ for the late type CQ Cephei (HD 214419, WN7+O9) and ~3 R¤ for the earlier V444 Cygni (HD 193576, WN5+O6). This
correlation of large radius with late type versus smaller radius with early type is
assumed to persist across all W-R sequences.

"As for ordinary stars, they are binned into
subclasses, in which higher values mean later (i.e. cooler) types. Calibration of
W-R stars in Galactic open clusters and in the Large Magellanic Cloud, yields a very
tight correlation between Mv and subclass [see van der Hucht et al.
1988]. ... With mean bolometric correction

for most W-R subclasses (Smith and Maeder 1988),
this implies that the total luminosity also increases systematically from earlier to later
WN or WC subtypes (e.g. Mbol» -11 for WN7
to -8 for WN4 or Mbol» -9 for WC8 to -8 for
WC5)" (Moffat et al. 1989, p. 230).

Thus the hottest W-Rs are the least brilliant; on
the H-R diagram, the W-R zone slopes downward to the left and narrows like a funnel as
shown in fig 2. (After Moffat et al. 1989, p. 233.)

Note that the positions
plotted in the figure refer to the cores of the stars [® sidebar].

All stellar observations are necessarily derived from radiation emitted
at a variety of physical depths within the atmospheres of the stars, and thus
represent a kind of average. We normally interpret them using the simplifying assumption
that they are all emitted at a certain optical depth (since the physical depth cannot be
known a priori), denoted t . Values of t = 2/3 and t = 1 are commonly used. For
stars with exceptionally strong stellar winds, such as Wolf-Rayet stars, however, the
region corresponding to an optical depth of t» 1 might well occur within the wind, far from the
"surface" of the star.

Occurrence

There are some curious occurrence trends associated with W-Rs. Few or no
WN8/9 stars are known to occur in WR+O binary systems, as opposed to nearly 60% of WN6/7
stars which do. Known orbital periods range from 1.6 days to 2886 days (Cherepashchuk
1992, p. 124). Shorter period binaries emit more X-radiation, probably from shock waves
formed as the O-star ploughs through the W-R wind, or, more particularly, by blobs of
matter ejected by the W-R object entering the photosphere of the O-star. "About half
of the well-investigated W-R+O binaries are eclipsing with the amplitude of photometric
variability DV = 0m.025 to 0m.5"
(Cherepashchuk 1992, p. 124).

WN8/9 stars rarely occur in clusters; again as opposed to WN6/7 stars which do. Noting
that particularly massive LBVs such as h Carinae occur in young
clusters together with WN7 types, Moffat et al. 1989 speculates (p.234) that WN8/9
stars may be derived from lower mass LBV progenitors.

Examples of Wolf-Rayet stars occurring in clusters
include two in the ~3 million year old NGC 6231 in Scorpius, one in NGC
2359 in Canis Major, one (HD 148937) associated with NGC 6164-65 in Norma,
and another (HD 192163) associated with NGC 6888 in Cygnus.

Mass and Luminosity

Wolf-Rayets are massive stars. Masses can generally only be estimated
from binary systems, which are commonly W-R+O systems. For such systems,
the mass ratio MW-R/MO, denoted q, increases for
cooler W-R subtypes. Thus, hotter W-Rs tend to be less massive with respect
to their O-type companions. Typical values for q range from ~0.2 for the
hottest (WC5, see below) types to ~1 for cooler (WN7) stars. The spectral
types of the O companions show no correlation with W-R subtype.

Typical masses are around 16 to 18 M¤
but the range is very great: from 5 M¤
to 48 M¤ or, in one case (WR 22, HD 92740), 77 M¤. Masses of the O star in W-R+O binary systems
range from 14 to 57 M¤, with
a mean of 33 M¤ (Cherepashchuk 1992, p. 123).

Variability

All subtypes of W-Rs show a correlation between
variability and luminosity similar to other supergiants, that is
increasing variability with higher luminosity which, in W-Rs, corresponds
to cooler stars. Thus, as they become hotter, they become more stable. The
microvariability time scale is in the order of one day.

Stellar Winds and Mass Loss

Not until the 1980s did it became clear that WR stars represent an evolutionary
phase in the lives of massive stars during which they undergo heavy mass
loss (Willis 1991). Their spectra indicate that the stars are embedded
in luminous and turbulent shells of ejecta flowing outwards at speeds
comparable to the expansion velocities of novae (Cherepashchuk 1992, p.
129, quotes values of n0 = 200 km
s-1 and n¥ = 2200 km
s-1), although, in the case of Wolf-Rayet stars, the "explosion"
is on-going; the expanding shell is being constantly fed with material
from the main body of the star, at rates of 10-5 to 10-4
M¤ per year.

Fig 2: NGC 3199, in the constellation Carina, which is the wind-blown
partial "ring" around the Wolf-Rayet (W-R) star WR 18 (= HD 89358),
the easternmost (leftmost) of the three bright blue stars near the
center of the 2MASS image. NGC 3199 and WR 18 are at a distance
of about 3.6 kpc (11,736 light years) from us.

[Atlas text and image courtesy of the Two
Micron All Sky Survey (2MASS), a joint project of the University
of Massachusetts and the Infrared Processing and Analysis Center/California
Institute of Technology, funded by the National Aeronautics and
Space Administration and the National Science Foundation.]

"Overall, observations indicate that WR
winds are especially strong, and even optically thick to continuum scattering
by electrons. Notably, inferred WR wind momenta M-dot.v¥
are generally substantially higher than for OB stars of comparable luminosity,
placing them well above the OB-star line in the WML relation. In fact, the
inferred ratio of the wind momentum to that of stellar luminosity, hº M-dot.v¥/(L/c),
is typically much above unity, sometimes as high as h
= 10 to 50" (Owocki 2000).

"Among the W-R stars, the luminous WNL subtypes
(especially WN8) are the most variable, probably as a consequence of blob
ejection in [their very strong stellar winds.] The underlying mechanism
which triggers this ejection is possibly related to wind instabilities and
may thus be quite different from the source of variability in luminous supergiants
or LBVs in quiescence, where photospheric effects dominate" (Moffat
et al. 1989, p. 229).

Some of the mass loss appears to occur as blobs
of matter being ejected in all directions; if there is a preference for
the equatorial plane it is not obvious from current observations. The presently
favoured triggering mechanism for blob ejection in W-R stars is simply stochastic
[random] instabilities in the stellar wind although there remains the problem
that the fast winds appear to be more stable than slower winds, contrary
to theory.

Cherepashchuk 1992 estimates that up to 80% of
the mass lost by W-R stars may be in the form of blobs ("dense and
compact clouds of matter" p. 127).

Interpretation

Composition

The Wolf-Rayet progenitor stars and formation processes are not yet clearly
understood. The presence of ‘core material’ in their surface
layers suggests two formation scenarios: extensive mixing of the inner
and outer layers by some means, or uncovering of the convective core of
massive stars through deep erosion of their outer layers.

Various mixing mechanisms have been suggested, including differential
rotation in fast spinning massive stars and large increases of the convective
core size ("convective overshooting," see Langer 1990 for more
detailed discussion and references).

In the second scenario, Wolf-Rayet stars are
essentially the naked cores of massive stars which have sloughed off their
outer layers. The progenitor stars might be red supergiants, luminous blue
variables or possibly binary stars which have lost their outer envelope to
a close companion through Roche-lobe overflow. While the last possibility
may be relevant in some cases (many WR stars are binaries or higher
multiples; see table 1) it cannot be considered a general solution since
many Wolf-Rayet stars are believed to be single.

Progenitors

It appears that W-Rs can evolve from any sufficiently massive star. It
is feasible that the most massive stars, ~120
M¤, may lose their original envelope
during H burning, evolving directly from Of to W-R stages (Maeder 1989,
p.16), or perhaps via a short-lived intermediate Ofpe/WN9
phase.

Theoretical models for main sequence stars – even those as massive
as 100 M¤ – suggest that they can evolve to cool
surface temperatures after core hydrogen is exhausted, provided mixing
is restricted (Langer 1990 proposes that molecular weight gradients might
impede convection).

A cool surface may initiate a high mass loss rate.
-- Langer doesn’t actually say this

Red supergiants have the requisite cool surface
and those more massive than about 50 M¤
may be Wolf-Rayet progenitors. Whether some of them evolve also through
a luminous blue variable (LBV) phase, and whether this would be before or
after the red supergiant phase, is unknown, although stellar evolution in
this part of the H-R diagram almost certainly encounters the Humphreys-Davidson
Limit. WN8 stars are close to LBVs on the H-R diagram and possess some
common features, namely a high degree of variability, narrow emission lines,
and high luminosity. On the other hand, the time scales of the variations
are quite different: in the order of one day for WN8 stars as opposed to
ten days or more in quiescent LBVs. In W-R stars the variability is a wind
phenomenon; in LBVs and red supergiants it is atmospheric.

WN7 stars are as luminous but hotter and more stable, perhaps on account
of being farther from the H-D limit.

Stars around, or just below, 40 M¤
may become exotic red supergiants – perhaps OH/IR stars – and
then W-Rs. Those well below ~40 M¤
probably do not encounter the H-D limit and probably do not become W-Rs
either.

Wolf-Rayet stars which do evolve from LBVs must become Wolf-Rayet stars
almost immediately at central helium ignition (since the LBV phase lasts
only a very short time; in the order of 105 years).

Evolution

Once Wolf-Rayet stars are formed, their subsequent
evolution is dominated by their mass loss, since "the mass-loss time-scale
is comparable to the [life time of the star] which means that the internal
structure and surface properties of the star are neither those of the more
massive star nor those of a main sequence star of lower mass.... In some
cases most of the hydrogen-rich outer envelope of a star may be lost
leaving a helium-burning star close to the helium main sequence" (Tayler
1994, p.176).

"All of the above trends, together with
the high mass loss rates, favour subtype evolution from cooler to hotter
subtypes within each sequence. Our interpretation is as follows: all W-R
stars start as WNL, the most massive, luminous and least evolved of all
subtypes. As the strong wind peels off the outer layers, the surface abundance
gets more exotic with time and the core radius shrinks. Transfer from WN
to WC, basically a surface phenomenon, will occur at a given WN-subtype
which depends on the initial metallicity Z.... In short, extreme mass loss
forces W-R stars to evolve downwards (fainter) and to the left (hotter)
in the H-R diagram, more or less towards the He-ZAMS. This is no surprise
since W-R stars are helium burning, with hydrogen-rich surface impurities
that diminish with time" (Moffat et al. 1989, pp.232-233).

The stars showing broadened emission lines are supposed to occupy the
left side of the W-R ‘funnel’ (marked B in fig. 3) and thus
are hotter for similar luminosity. It is presumed they may have evolved
from more massive progenitors.

Once a star enters the funnel, it does not return red-ward (to the LBV
region).

Successors

"After the WNL stage, W-R evolution progresses downwards in the
HR Diagram, as the stars lose more mass, shrink in size and head for the
He-ZAMS" (Moffat et al. 1989, p.229).

"Just as massive supergiants show increasing variability as they
approach the Humphreys-Davidson instability limit (horizontally in the
HR Diagram), so the W-R stars show decreasing variability as they recede
from the H-D limit (at first horizontally into the WNL domain, then, with
their high mass loss rates, plunging irreversibly downwards as ever hotter,
smaller and fainter, strong-line W-R stars)" (Moffat et al.
1989, p. 229).

Eventually the W-R star will run out of fusible material, ending its
life as an early WC (WO) star in a type
Ib supernova.

Examples

Gamma2 Velorum

No WR star is so easily found as the bright naked eye (1m.7)
Gamma2 (g2) Velorum,
a famous visual multiple comprising a brilliant Wolf-Rayet
(WC8) primary, possibly the nearest such star to us, and an unrelated
magnitude 4, type B companion (g1
Velorum). There are two wide companions, the more distant itself having
a very close companion bringing the complement to five. Finally, the primary
itself is a spectroscopic binary, the unseen component being a giant type
O7 star.

Others

Further examples of Wolf-Rayet stars include two
occurring in the ~3 million year old NGC 6231 in Scorpius, one in NGC 2359
in Canis Major, HD 148937 (associated with NGC 6164-65 in Norma, about
4000 light years distant, see Malin 1993 p. 160) and HD 192163 (associated
with NGC 6888 in Cygnus, also about 4000 light years distant, spectral
type WN).